Alloy 28 Stud Bolt - Strength and Durability for Critical Applications

Alloy 28 (UNS N08028) stud bolts occupy the narrow band where 316L corrodes too fast and duplex 2205 is simply the wrong alloy: hot sulphuric and phosphoric acid service. This iron-nickel-chromium super-austenitic — about 27% Cr, 31% Ni, 3.5% Mo and 1% Cu — keeps flanged joints tight in reducing media, provided the rest of the joint is specified as strictly as the studs, starting with compatible flange gaskets.

What Alloy 28 Actually Is

Alloy 28 is designated UNS N08028 and also appears as DIN 1.4563. It is best described as a super-austenitic iron-nickel-chromium alloy rather than a stainless steel, because the nickel content sits around 31% and the alloying strategy differs from the 300-series.

The composition is roughly 27% chromium, 31% nickel, 3.5% molybdenum and 1% copper. Chromium provides the passive layer, molybdenum resists pitting, copper improves resistance to sulphuric acid, and the high nickel stabilises the austenite and protects against stress corrosion cracking.

• UNS N08028 (DIN 1.4563): a super-austenitic iron-nickel-chromium alloy, not a 300-series stainless.
• Built for sulphuric and phosphoric acid service where 316 and even 904L corrode too quickly.
• Duplex 2205 wins on strength and chloride pitting; Alloy 28 wins on reducing acids and on resistance to chloride stress corrosion cracking.
• A high-nickel alloy priced accordingly — specify it where the medium genuinely rules out cheaper grades.

Alloy 28 sits in the same family as 904L and the 6% molybdenum super-austenitics, and the practical selection question is which of those three fits the specific acid. That is a question about the medium, not about which alloy is best.

Why Sulphuric and Phosphoric Acid Are Difficult

Sulphuric acid is a reducing acid in dilute and moderate concentrations. That matters because conventional stainless relies on an oxidising passive film, and a reducing acid removes that film rather than maintaining it.

The practical consequence: 316 corrodes rapidly in sulphuric acid at moderate concentration and temperature — exactly the operating window where a chemical plant spends much of its time.

Copper additions promote a different surface behaviour in sulphuric acid, which is why Alloy 28 and 904L both carry around 1–1.5% copper while the 300-series carries none.

Phosphoric acid adds a second problem: it frequently carries chlorides and fluorides as impurities from the phosphate rock. That combination — acid plus halide plus temperature — is where standard austenitics fail by both general corrosion and stress corrosion cracking.

Note: typical service conditions only; final selection depends on acid concentration, temperature and impurity levels.

The table shows why the question of which alloy is most corrosion resistant is the wrong question. 2205 duplex is far stronger and far better in chloride pitting, yet it is the wrong choice for hot sulphuric acid. Alloy 28 is the reverse: modest strength, but it survives media that destroy duplex.

Mechanical Properties and Specifying Alloy 28 Studs

Alloy 28 is an austenitic alloy, so its yield strength is comparable to 316 rather than to duplex — roughly half the yield of 2205. Its advantage is corrosion resistance, not strength, and a joint designed around duplex will need larger studs if it moves to Alloy 28.

It is available as bar and hence as stud bolts across a wide size range, and it is readily cold formed and machined — which matters because Alloy 28 fasteners are often one-off or low-volume items for a specific piece of process equipment. Welding is possible but requires matching filler and the same cleanliness rules as any high-alloy material: a contaminated weld in an acid line is the first place to fail.

• State UNS N08028 rather than a trade name alone, since trade names drift between suppliers.
• Confirm the product form — bar stock for machining or finished studs — because the testing requirements differ.
• Ask for the actual heat analysis, particularly chromium and molybdenum; a heat at the low end of the range behaves noticeably worse.
• Specify the surface condition: pickled and passivated is the baseline for acid service.

Finally, match the nuts. A high-alloy stud with a 316 nut simply moves the corrosion to the nut — the cheaper part, but also the one that decides whether the joint can be dismantled after five years in service. The same logic applies across the whole joint, from the pipe flanges themselves down to the bolting.

Where Alloy 28 Studs Are Specified

Alloy 28 studs are specified where the medium rules out cheaper grades.

• Sulphuric acid plants — heat exchanger flanges, acid coolers, piping joints and pump casings where concentration and temperature fall outside what 316 or 904L tolerate.
• Phosphoric acid production and fertiliser plants, where chloride and fluoride impurities raise corrosion rates well above pure-acid data.
• Pickling and surface treatment lines, where mixed acids at temperature attack both the equipment and its fasteners.
• Offshore and refinery service where chlorides and wet acid vapours coexist and the cost of a leak justifies a higher grade from the outset.

Alloy 28 is not immune to chloride stress corrosion cracking. It resists it far better than 316, but the resistance comes from the nickel content and it declines at high temperature and high chloride concentration.

For very high chloride with oxidising conditions — bleaching, seawater with chlorine injection — the 6% molybdenum grades or a nickel-based alloy are the correct step, not Alloy 28. Thermal expansion and galling also deserve attention: the high-nickel austenitic structure galls readily during tightening, so anti-seize and controlled torque are not optional extras on these fasteners.

Frequently Asked Questions

What is Alloy 28 used for?

Mainly sulphuric and phosphoric acid service, including fertiliser plants and pickling lines, where 316 and even 904L corrode too quickly. It is also used where chloride impurities in the acid raise the risk of stress corrosion cracking.

Is Alloy 28 better than duplex 2205?

Not universally. Duplex is about twice as strong and better against chloride pitting; Alloy 28 is markedly better in reducing acids such as sulphuric acid. The medium decides, which is why the acid concentration and temperature must be stated.

What is the difference between Alloy 28 and 904L?

Both are super-austenitic alloys with copper additions for sulphuric acid service. Alloy 28 carries more nickel, which improves resistance to chloride stress corrosion cracking, so it is preferred where the acid also carries chlorides.

How should Alloy 28 stud bolts be specified?

State UNS N08028, confirm the product form, request the actual heat analysis for chromium and molybdenum, and specify pickled and passivated surfaces. Pair the studs with nuts of matching grade so the joint stays dismantlable.